The invention resides in the area of measurements of electrical quantities, such as resistance or impedance. In particular, it is directed to a method and apparatus for measuring accurately the resistance or impedance of an element under measurement at the power frequency by the use of an AC current comparator.
AC comparator bridges are used in a variety of applications including a field of electrical measurements, i.e., measurement of such electrical quantities as current, resistance, impedance, capacitance voltage etc. In the case of resistance measurement, an article entitled xe2x80x9cA Direct-Reading AC Comparator Bridge for Resistance Measurement at Power Frequenciesxe2x80x9d, So and Moore, IEEE Transactions on Instrumentation and Measurement, Vol. IM-29, No. 4, December 1980, pp 364-366 describes a current comparator technique for the measurement of resistance (impedance) at 50-60 Hz with high accuracy. In the technique described, the current in the unknown resistor is compared, using the current comparator, to the current in a reference resistor for the in-phase component and to the current in a reference capacitor for the quadrature component or phase defect.
A problem in the precise measurements of the ratio of low-value impedances is the effect of voltage drops in the connecting leads. This is overcome by applying four-terminal measurements. In the previous developments, including the technique described in the above article, active circuits are used to eliminate the effects of lead and winding impedances. Elimination of the effects of the leads at the high voltage ends of the unknown and reference resistors is accomplished by equalizing the potential terminals of the both resistors through an amplifier. At the low voltage end, the effects of lead and current comparator winding impedances are eliminated by an active circuit injecting a correction current into the low-end potential terminal of the reference resistor, so that the effective currents in the unknown and reference resistors are equal to those which would be obtained when the same voltage is applied to both resistors.
For most practical purposes this correction circuit is sufficient. However, it requires a resistor in the active circuit which is equal in magnitude to the reference resistor. The matching of the two resistors becomes more critical with an increase in the ratio of the voltage drop across the lead and the winding impedance and the voltage drop across the unknown resistor. This is inconvenient, especially if the reference resistor is to be changed to allow different ranges of the bridge. As mentioned earlier, prior techniques also require a reference capacitor in order to take into account the quadrature component or phase defect of the unknown resistor.
The present invention addresses these problems and presents an improved power-frequency current-comparator-based four-terminal resistance bridge, which does not require a correction circuit with matching resistors and does not require an external reference capacitor for quadrature current compensation, such as described in the above referenced article.
An adaptation to computer-controlled operation is facilitated by the simple, yet effective bridge structure.
An article entitled xe2x80x9cA Computer-Controlled Current Comparator-Based Four-Terminal Resistance Bridge for Power Frequenciesxe2x80x9d, So and Djokic, IEEE Transaction on Instrumentation and Measurement, Vol. 50, No. 2, April 2001, pp 272-274 describes the present invention. The inventor of the present invention is one of the coauthors of the article.
In accordance with an aspect, the invention relates to a bridge for measuring accurately an electrical parameter of an unknown element. It compares the unknown element and a reference element and uses an ampere-turn balance in a current comparator to derive the electrical parameter in relation to the reference element.
In accordance with a further aspect, the invention is directed to a current comparator-based bridge for measuring accurately an electrical parameter of an unknown element. It includes a current source for serially flowing a first electrical current through the unknown element and a current comparator and a buffer circuit bridging the unknown element and a reference element for flowing a second electrical current through a reference resistor, a sensing resistor and the current comparator. The bridge further includes a quadrature circuit for generating a quadrature component of the second electrical current through the current comparator and a controller circuit for controlling and sensing an ampere-turn balance in the current comparator to derive the electrical parameter of the unknown element in relation to the reference element.
In accordance with a further aspect, the invention is directed to an AC resistance bridge for measuring accurately an impedance, equivalent capacitive component or equivalent phase defect angle of an unknown element at a power frequency. The bridge comprises a current comparator having a plurality of windings, and a current source for supplying a first electrical current of the power frequency to a first AC circuit comprising the unknown element and a first winding of the current comparator. The bridge also includes a second AC circuit comprising a reference element, a current sensing element and a second winding of the current comparator, buffer amplifiers connecting the first and second AC circuits for supplying a second electrical current through the second AC circuit, and a quadrature circuit connected to the current sensing element to generate a quadrature current through a third winding of the current comparator. The bridge further comprises a sensing module for monitoring an ampere-turn balance in the current comparator to derive the impedance, equivalent capacitive component or equivalent phase defect angle of the unknown element in relation to the reference element.